1. Field of the Invention
The present invention relates to the technical field of Universal Serial Bus (USB) and, more particularly, to an USB composite device.
2. Description of Related Art
Currently, electronic products are developing toward the tide of high-speed transmission due to quickly improved electronic technologies. However, for a personal computer (PC), the widely used universal serial bus (USB) has developed from USB1.1 to USB2.0. USB2.0 standard can have the transmission speed up to 480 Mbits/s about 40 times faster than 12 Mbits/s of USB1.1 standard. Also, it uses a pair of differential signals USBD+ and USBD− for transmission. Since the transmission speed up to 480 Mbits/s and the action of transmitting and receiving the pair of differential signals USBD+ and USBD− use analog techniques, Intel cooperation accordingly defines an USB 2.0 Transceiver Macrocell Interface (UTMI) interface signals for analog-specialized companies to design USB 2.0 Transceiver Macrocell (UTM) while other IC design companies design the remaining functions in USB 2 standard. Thus, specialization is obtained to speed the distribution of USB 2.0 products.
However, when a host 110 with USB 2.0 socket wants to connect plural USB devices, due to the limit of USB socket number, as shown in FIG. 1, the host 110 has to link a hub 120 first and downlink ports 122 and 123 of the hub 120 and then connect to an USB printer 130 and an USB scanner 140. As shown in FIG. 1, such a configuration requires six physical layers. Thus, the conventional skill requires plural physical layers, which increases cost. In addition, in this case, the hub 120 requires three physical layers implemented by analog techniques, and thus its IC yield is affected.
Therefore, it is desirable to provide an improved USB composite device to mitigate and/or obviate the aforementioned problems.